The hijackers who seized Trans World Airlines Flight 847 Friday released 64 of their remaining hostages here yesterday and today and set a new deadline of 5 a.m. today EDT for their demands to be met or "a price will be paid."

All the hostages still on board were believed to be Americans, but it was unclear how many remained of the original 145 passengers and eight crew.

A State Department spokesman said that there have been reports from some of the released passengers and crew members that eight or nine passengers were taken off the plane by Shiite militiamen when the plane was on the ground in Beirut early Saturday. The spokesman said that those reports are unconfirmed.

The heavily armed hijackers, now reported to number about 10, are demanding, among other things, the release of hundreds of Shiite militiamen held by Israel.

"If Sunday at 10 a.m. Algeria time our Arab brothers jailed in Israel are not liberated, we will leave Algiers for an unknown destination, and a price will be paid," the hijackers said in a statement carried by the official Algerian news agency APS.

"We are not prepared to release those detained by us, and we threaten to kill them one after one if there is no response to our demands," the hijackers said earlier, according to Algiers radio, monitored in London.

Sources in Washington said members of the Delta commando unit of the Army's Special Forces based in Fort Bragg, N.C., were en route to the area in case they were needed to storm the plane. U.S. officials noted, however, that the White House was taking a low-key approach to the matter and preferred to rely on the Algerians and the Red Cross to help reach a solution.

A five-member International Committee of the Red Cross team arrived in Algiers late last night led by Jean Hoefliger, who is in charge of the Middle East section of the Red Cross and played a role in arranging a recent exchange of Israeli and pro-Syrian Palestinian prisoners.

"We have been approached and requested to come here by the U.S. government, the people in the plane and by the Algerian authorities in order to see what can be done," Hoefliger told reporters before a meeting with U.S. Ambassador Michael Newlin. "Our main concern is for the people on board the plane."

Shortly before 2 a.m. today, another three American hostages were released by the hijackers in an apparent gesture to the negotiating team from the Red Cross that had landed in Algiers a few hours earlier from Geneva.

The hijackers had freed 61 captives here yesterday. The group of three hostages was set free after the Red Cross negotiator went out to the airplane to talk to the hijackers.

The airliner was parked at a remote, floodlit corner of the airport, and Algerian security forces patrolled a road nearby.

The hijackers, originally thought to number only two Lebanese Shiite Moslems, may have been joined by several more terrorists during the plane's second stop in Beirut early yesterday. Shiite Amal militiamen were in control of Beirut Airport when the airliner landed, and several heavily armed men were seen talking to men from the plane on the blacked-out runway before it took off for Algiers again.

Algerian police said they believed that about 10 hijackers were aboard. One of the freed passengers, Christine Cook, 34, a Boston psychologist, said about six more hijackers boarded the plane in Beirut.

The hijackers were joined in Algiers by Ali Atweh, their Lebanese colleague who was arrested by Greek authorities Friday and then flown to Algiers today in an exchange for Greek passengers on the TWA Boeing 727.

Cook said that at the beginning of the hijacking two armed men who boarded in Athens were running up and down the aisle, frightening the passengers. She said they were armed with machine guns, handguns and grenades

"I haven't slept, and I was terrified, but the hijackers were not so brutal with us." she said. She said she did not see the man who was shot in Beirut and whose body was thrown out of the plane.

The hijacked airliner returned to Algiers for the second time shortly before 8 a.m. yesterday morning after its second stop in Beirut. It had started in Athens, bound for Rome with 145 passengers and eight crew. TWA said the hijackers released 19 hostages during its first stop in Beirut and 21 others in Algiers. One passenger was slain. The release of the hostages so far left it unclear how many still were being held.

Reports that more than 40 passengers and three crew remained aboard the hijacked jetliner did not take into account the reports that several of the passengers may have been taken by Shiites in Beirut, and it is unclear what happened to all those freed at the Shiite-controlled airport in Beirut.

The hijackers released 51 passengers yesterday morning, including American women who were members of the flight crew, Greek passengers and children. Many of these were flown to Paris. Hours later they released 10 men, most of them elderly Americans.

Another released passenger, Penny Bamford, 34, a nurse from Lexington, Ky., said, "It was the most terrifying experience I have ever had."

Paula Sukeforth, 45, of Lexington, Mass., said of the gunmen, "They were kind and considerate towards us. They praised Algeria, saying it worked hard for our freedom.

"At the beginning of the hijacking a couple of people were beaten up. But afterwards they just told us to keep our heads between our knees. It lasted for many hours. We couldn't sleep. We couldn't talk to each other. I'm glad I'm here now because I was terrified," she said.

Gail Zillgitt, a 20-year-old student from Indianapolis, Ind., told reporters that she was returning from a five-week vacation and was traveling with another woman and her boyfriend. "He's still on the plane, and we're very worried about him."

"The worst thing was we had to put our heads down for hours," she added. "It was very hot in the plane, but the hijackers gave us enough food and enough water. They were relatively calm, and they had a certain sense of humor. I was scared, but they were nice and tried to help. At the same time they always made sure we obeyed them.

"They criticized American policy in the Middle East and said they wanted as much money from the U.S. as Israel gets."

Douglas Porter, of Geneva, Ill., one of a group of about 10 middle-aged male hostages released this evening, said that discipline aboard the plane had been very strict and that passengers had to raise their hands to go to the bathroom.

Ken Lenham, of San Francisco, said: "They let us off the plane because I suppose they thought that we were old and useless."

The release of the hostages yesterday followed the intervention of two Algerian officials who boarded the aircraft about 90 minutes after it arrived in Algiers for the second time early yesterday.

In Washington, national security adviser Robert C. McFarlane, John Poindexter, his deputy, and Robert Sims, White House deputy press secretary, were monitoring developments through a State Department task force.

"The president is appreciative of the Algerian government," Sims said, adding, "The matter is in the hands of the Algerian government."

President Reagan remained at Camp David with no plans to return before later today, according to a White House spokesman. Officials said he was being kept informed of developments.

The Algerian ambassador to the United States, Mohammed Sahnoun, met yesterday with Secretary of State George P. Shultz at the State Department. He was understood to be in frequent contact with the State Department about the crisis.

Ambassador Newlin said yesterday that the United States would not negotiate or make concessions to the hijackers. Speaking to reporters at the airport, he said the Algerians were in charge of the negotiations.

"The United States will not negotiate with terrorists nor make concessions. This has been our position, and we will stick to it," he said.

Noting the number of hostages that have been released since the beginning of the hijacking, he said, "This is considerable progress." He expressed his "personal appreciation for the efforts made by the Algerian government."

Israeli Defense Minister Yitzhak Rabin consulted Saturday with U.S. officials on the hijacking, Israeli radio said, Reuter reported from Tel Aviv.

The report, which could not be confirmed, said Israel had been planning to free 700 Shiites soon, in line with previous statements that it had no intention to keep them prisoner permanently.

In Washington, an Israeli Embassy spokesman said, "We are in close consultations with American authorities dealing with the case."

The identity of the man shot by the hijackers in Beirut early yesterday still had not been made public. No reporters were allowed into the morgue, and a search of other hospitals in east and west Beirut was futile. U.S. Embassy officials said last night that the body of the slain passenger was now in a hospital in a Christian-controlled area.

A worker at a hospital morgue said that he had carried in the body of a passenger brought by the Lebanese Red Cross from the airport. Amal officials at the hospital emergency room said the body was at the Sahel Hospital. "We would not be foolish enough to bring an American body to an American hospital," one of them said. "We would keep it somewhere else. Maybe we could use it in getting thousands of our people back."

A soldier from the Lebanese Army's 6th Brigade said the body had been brought in at 4:30 a.m. One irate Amal hospital worker, who said he spent all day Friday carrying the bodies of Lebanese killed in shelling and from a car bomb explosion, asked why a reporter should worry so much about "one American, when there are hundreds of Lebanese killed every day."

The Shiites were clearly in control of Beirut's airport when the airliner touched down for the second time early yesterday.

Tough-looking members of the Amal militia security unit, considered among the most disciplined of Amal fighters, arrived at the airport before the plane touched down yesterday morning. After it had landed, it turned off its lights. Armed men then ran out and spread in a long line near the aircraft.

A security official, Capt. Mohammed Hamiyeh, was involved in the negotiations conducted from the airport control tower. When he told the hijacker he was part of the state security system, the hijacker responded: "There would not be a problem if I recognized this state."

Then the conversation took a dramatic turn. Getting angry and raising his voice, the hijacker said: "Here, listen to the sound of bullets."

The voice of the American pilot came on: "They just shot a passenger."

When the pilot begged the hijacker to give the negotiators time to figure out what he wanted, he screamed: "I'm not joking with you. I give you five more minutes, and then the Amal envoy must be here."

After Amal officials met with the hijackers, the plane took off for Algiers again.